The present invention relates to 3D color printing.
Rapid prototyping or rapid manufacturing processes are manufacturing processes which aim to convert available three-dimensional CAD data directly and rapidly into workpieces, as far as possible without manual intervention or use of molds. Most printers are single color printers, but following the trends in printing, consumers will demand high quality and affordable color printing.
A challenge faced in color 3D printing nowadays is the building material used. In the common design of a low-cost thermoplastic 3D printer, only a single heat extruder is used to provide energy for the melting, and some modifications are made in possible solutions. Multi-extruder 3D printers are available, but provides unrealistic output because the melted plastic cools down as soon as it touches the supporting bed and gets solidified. These multi-extruder 3D printers cannot mix solidified droplets to get a continuous full color object as each droplet is too large (much larger than the size of ink droplets in a desktop inkjet printer). Some color 3D printers also try to mix materials in color before extruding them, but it is hard to mix melted thermoplastics, the common building material, of different colors to a great extent since they melt at above 200 degree Celsius and cool rapidly if not insulated.
Conventional rapid prototyping processes can be divided into two groups: laser-based processes and processes without use of a laser. One approach is stereolithography (SLA) where a liquid composition of a radiation-curable polymer is hardened layer-by-layer by using a laser. If color is desired on a workpiece produced by SLA, the piece is then subsequent colored on the surface. This is complicated and time-consuming. Another process is Selective Laser Sintering (SLS), in which, by analogy with the SLA, a pulverulent raw material, e.g. a thermoplastic or a sinterable metal, is sintered selectively layer-by-layer by a laser. Again, the first step of this process can only produce single-color, or non-specifically colored, 3D objects. The same issue applies to coloring in a third laser-based process, “laminated object manufacturing”, in which layers of a paper web or plastics foil provided with adhesive are adhesive-bonded to one another and cut by a laser. One example of subsequent coloring of an object is described in U.S. Pat. No. 6,713,125.
A conventionally known 3D printing process which can also be used for the production of color objects is the UV ink-jet process. In this three-stage process, a pulverulent material is applied in thin layers, a UV-curable liquid is printed in the form of the respective layer of the subsequent three-dimensional product onto the said layers, and finally the printed layer is hardened by using a UV source. These steps are repeated layer-by-layer.
In WO 2008/077850 variously colored liquids are provided with hardener that are mixed in a chamber directly upstream of the printing process. Selective coloring is thus possible. However, no sharp color transitions are possible, because of the mixing chamber. This type of process moreover lacks sharpness at the limits of the hardening process, and this can reduce surface smoothness, and can sometimes lead to non-uniform coloring. In WO 2001/26023, two printing heads are described with variously colored hardener compositions, giving different elasticity properties in the products. However, the number of colors described is not more than two.
In a process in accordance with WO 2009/139395, similar to 3D ink-jet printing, a colored liquid is applied layer-by-layer and printed selectively with a second liquid which leads to a curing reaction with the first liquid. This type of process can only produce a structure of layer-by-layer colors, except in so far as mixing can occur between the uncured layers of liquid.
In a process described in US 2004/0251574, the print of the thermoplastic is followed by selective printing with an ink. This process has the advantage of permitting highly selective printing. However, this process has the disadvantage that it is impossible to achieve uniform color definition or bright coloring, since there is no possibility of achieving uniform penetration of the ink into the composite made of the (ceramic) powder and of the binder.
In U.S. Pat. No. 6,401,002, various liquids are used with different inks and the binder. The said liquids are either applied separately dropwise or combined by way of connecting lines in a nozzle upstream of the dropwise application process. The person skilled in the art is aware that neither procedure gives ideal color definition. In the former, the mixing of the inks takes place in viscous liquids on the surface. This mixing is therefore rarely complete. In the second procedure, pressure differences in the connecting lines can lead to extreme color variation.
Among printing processes for the production of three-dimensional objects, the process that is most economical in use of materials and that is also most advantageous in terms of design of machinery is the fused deposition modeling (FDM) process. This involves an extrusion-based, digital manufacturing system. There are also other known processes that are substantially analogous with slight differences, for example fused filament fabrication (FFF), melted extrusion manufacturing (MEM) or selective deposition modeling (SDM). In the FDM method, two different polymer filaments are melted in a nozzle and are printed selectively. One of the materials involves a support material which is needed only at locations above which an overhanging part of the 3D object is printed and requires support during the subsequent printing procedure. The support material can be removed subsequently, e.g. via dissolution in acids, bases or water. The other material (the build material) forms the actual 3D object. Here again, the print is generally achieved layer-by-layer. The FDM process was first described in U.S. Pat. No. 5,121,329. Coloring is mentioned in general terms in US 2002/0111707, but is not described in any detail. In the 3D color-printing method in accordance with U.S. Pat. No. 6,165,406, separate nozzles are used for each individual ink. There are therefore only very restricted possibilities for ink mixing, and the color effect achieved becomes very simple. In the FDM variant described in U.S. Pat. No. 7,648,664, variously colored build materials are used in granulate form, melted separately from one another, and mixed with one another in accordance with color requirement by an intervening extruder, before application as print. This method requires very complicated apparatus, and many advantages of FDM are lost. U.S. Pat. No. 6,129,872 describes a process in which the build material is melted in a nozzle and various colorant mixtures are metered selectively into the melt at the end of the nozzle. However, this leads to inadequate mixing and does not give clean color definition.
US 2010/0327479 describes a process in which a plurality of colored filaments are combined in a microextruder and are continuously extruded therein to give a new colored filament, which is then passed onward into the printing head for application as print. This process requires very sophisticated and complicated apparatus. The achievable color range is moreover subject to restriction resulting from the number of filaments. In an alternative embodiment, the variously colored filaments can also be conducted directly into the printing head, and mixed there.
United States Patent Application 20140134334 discloses a 3D extrusion print process for producing multicolored three-dimensional objects. The process is based on coating, upstream of the printing head, of the polymer strand used for producing the actual object, and on fixing of the coating upstream of entry of the polymer strand into the printing head. Downstream of the extrusion process in the printing head, the coating remains predominantly at the surface of the extruded strand.